Diagnosing brain injury, especially mild damage, is based largely on subjective symptoms such as irritability and forgetfulness. Imaging tests such as CAT scans do not help, and neurological function tests are not very useful without baseline information.

Researchers led by Air Force Lt. Col. Dr. Michael Xydakis checked all troops brought for treatment from roadside bombs and other explosions to the Air Force Theater Hospital in Balad, Iraq, during the last three months of 2005.

The study focused on the 210 surviving U.S. troops who were evaluated for eardrum rupture and loss of consciousness. Of those, 35 percent had ruptures and 36 percent had lost consciousness; the two were closely linked.

Those with ruptured eardrums had a nearly threefold greater risk of concussive brain injury.

The eardrum is only half a millimeter thick -- thinner than a contact lens -- and ruptures easily, Xydakis said. It is only half an inch from the brain, so "whatever hits the eardrum is going to hit the brain," such as the pressurized shock wave that follows an explosion, he explained.

Eardrum ruptures usually heal by themselves but occasionally need to be reattached. Concussive brain injuries have a variety of treatments, depending on severity and symptoms. Early detection is thought to improve the chances of avoiding permanent damage.